More particularly, a helicopter is piloted while monitoring numerous instruments on a control panel, which instruments for the most part represent the operation of the power plant and of the aircraft. For physical reasons, there are numerous limits that the pilot needs to take into account at each instant while flying. These various limits generally depend on the stage of flight and on external conditions.
Most presently-manufactured helicopters are fitted with a power plant that possesses one or two free-turbine turbine engines. Power is then taken from a low-pressure stage of the free turbine, which stage is mechanically independent of the assembly comprising the compressor and the high-pressure stage of the turbine engine. The free turbine of a turbine engine rotates at a speed lying in the range 20,000 revolutions per minute (rpm) to 50,000 rpm, so a speed-reducing gearbox is needed for the connection to the main rotor since its speed of rotation generally lies in the range 200 rpm to 400 rpm: this gearbox is the main power-transmission gearbox (MGB).
Thermal limits on the turbine engine and torque limits on the main gearbox serve to define three normal utilization ratings for the turbine engine:                a take-off power rating usable for five to ten minutes and corresponding to a level of torque for the main gearbox and a level of heating for the turbine engine that can be accepted for a limited length of time without significant degradation: this is referred to as the take-off power (TOP) rating;        a maximum continuous power rating during which the capabilities of the main gearbox and the capabilities that depend on maximum acceptable continuous heating upstream from the high pressure blades of the first stage of the turbine are never exceeded: this is referred to as maximum continuous power (MCP) rating; and        a maximum transient power rating that might optionally be limited by regulations: this is referred to as the maximum transient power (MTP) rating.        
There also exist emergency excess supercontingency ratings on multi-engine aircraft that are used in the event of a turbine engine breaking down:                the first emergency rating during which the capabilities of the inlet stages of the main gearbox and the temperature potential of the turbine engine are used to the maximum: this is referred to as a super emergency power (PSU), and it can be used for up to thirty consecutive seconds, at the most, and it can be used three times in one flight. If the super emergency power (PSU) has been used, then the turbine engine must be removed and overhauled;        the second emergency rating during which the capabilities of the inlet stages of the main gearbox and the potential of the turbine engine are used very largely: this is referred to as a maximal emergency power (PMU) and can be used for a maximum of two minutes after the super emergency power (PSU) or of two minutes and thirty seconds consecutively, at the most; and        the third emergency rating during which the capabilities of the inlet stages of the main gearbox and the thermal potential of the turbine engine are used without damaging them: this is referred to as an intermediate emergency power and it can be used for thirty minutes or continuously for the remainder of the flight after the turbine engine has broken down.        
The above-mentioned limits are generally monitored by means of three parameters: the speed of the gas generator, engine torque, and the temperature at which gas is ejected into the inlet of the free turbine, these parameters being respectively written Ng, Cm, and T4 by the person skilled in the art.
For each rating, the manufacturer defines thresholds beneath which the monitoring parameters must be maintained.
The pilot must then monitor these monitoring parameters in order to ensure that the defined threshold for the rating in use is not exceeded. Such monitoring becomes particularly difficult for a helicopter having at least two engines insofar as it is appropriate to use three distinct dials for each engine.
Furthermore, the most recent turbine engines are controlled and regulated by a control and regulator member that is fitted with a regulator electronic computer, referred to as a FADEC by the person skilled in the art, and that serves in particular to determine the position of the fuel metering device as a function firstly of a regulation loop including a primary loop based on maintaining the speed of rotation of the lift rotor of the rotorcraft and secondly of a secondary loop based on a setpoint value of the piloting parameter.
A FADEC regulator member also receives signals relating firstly to monitoring parameters of the turbine engine under its control, and secondly to monitoring parameters of important members of the rotorcraft such as the speed of rotation of the lift rotor, for example.
The FADEC regulator member then transits the values of the monitoring parameters to a first limit indicator that is arranged in the rotorcraft cockpit.
Document FR 2 749 545 in particular describes a first limit indicator that identifies from amongst the monitoring parameters of the turbine engines, which is the particular parameter that is closest to its limit. The information relating to the limits to be complied with is thus grouped together on a single display making it possible firstly to summarize the information and present solely the result of that summary so as to simplify the pilot's task, and secondly to save space on the control panel. This produces a “limiting parameter” selected from amongst the monitoring parameters of the turbine engine, i.e. the parameter having a current value that is the closest to its limit value. That is why such an indicator is sometimes referred to as a “first limit instrument” (FLI).
This first limit indicator thus enables the present value of the limiting parameter to be known at any instant. This reduces the pilot's workload considerably since the pilot need use only one measuring instrument and no longer needs to use six instruments for a twin-engine helicopter.
This first limit indicator is sometimes displayed on a first screen of a first equipment in a central display system, the first equipment being referred to as a vehicle engine multifunction display (VEMD).
Furthermore, this first equipment is capable of displaying on a second screen the temperature and the pressure of oil in the turbine engine(s), the temperature and the pressure of oil in a main gearbox, the voltage and the current delivered by an electricity generator, and the outside temperature.
In the event of the first equipment breaking down, e.g. its screen breaking down, the pilot no longer has a first limit indicator for avoiding exceeding the limits set by the manufacturer.
The pilot must then rely on auxiliary instruments in order to obtain the values of the monitoring parameters. More precisely, on a helicopter having two turbine engines, the pilot needs to monitor six conventional indicators in order to monitor the speed of the gas generator, the engine torque, and the temperature at which gas is ejected into the inlet of the free turbine, for each turbine engine. In parallel, the pilot needs to consult the flight manual in order to determine what threshold should not be exceeded for each of the monitoring parameters.
This increases the workload on the pilot at a time when flying conditions are degraded. The pilot is then requested to interrupt the current mission and to make a precautionary landing without a period of hovering flight. The landing procedure may be difficult if the helicopter is to land on a confined zone, e.g. on board a ship.
Consequently, in order to preserve the engines of a helicopter having at least two turbine engines in the event of a first limit indicator breaking down, the known architecture requires the pilot to cancel the current mission, and increases the pilot's workload by requiring the pilot to use conventional instruments in order to avoid exceeding the thresholds set by the manufacturer.